Filtration of fluids such as gases is often performed to remove particulate or disparate impurities from the gas stream in order to limit harmful emission of the impurities into the environment, or circulation back into the associated process. It is ordinarily desirable to maximize the surface area available for filtration so as to remove large amounts of undesirable contaminants from the fluid stream, while maintaining the operating pressure differential induced by the filter as low as possible to achieve long service life and minimize surface strain.
In one form of filtration that is typically referred to as interception the filter media functions in the nature of a sieve that mechanically entraps particles larger than the pore size inherent to the media. Larger particles are removed from the fluid stream by the openings in the filter media, with particles building on top of one another to create a filter cake that removes successively smaller particles.
More specifically, in a so-called “baghouse filter”, particulate material is removed from a gaseous stream as the stream is directed through the filter media. In a typical application, the filter media has a generally sleeve-like tubular configuration, with gas flow arranged so as to deposit the particles being filtered on the exterior of the sleeve. In this type of application, the filter media is periodically cleaned by subjecting the media to a pulsed reverse-flow, which acts to dislodge the filtered particulate material from the exterior of the sleeve for collection in the lower portion of the baghouse filter structure. U.S. Pat. No. 4,983,434, hereby incorporated by reference, illustrates a baghouse filter structure and a prior art filter laminate.
The most commonly used fibers for hot gas filtration are polyethylene terephthalate (PET), acrylics, meta-aramids, polyimides, polyphenylene sulphide, and glass, which are normally used as fiber mixtures. U.S. Pat. No. 4,295,868 describes a silicic acid glass fiber-based fabric coated with metal oxides to improve fiber strength. The patent mentions that glass fibers containing 95% or more SiO2 are considered to be too brittle to be used as filters. WO 86/00570 describes a layered filter fabric containing a separate glass layer and one or more organic fiber layers. The glass fiber used could be made out of C-glass, E-glass, or S-Glass. WO 87/01655 describes a filter medium containing a layer of high temperature fibers such as glass, attached to a fabric made of high temperature fiber such as glass, so as to achieve good flex resistance. The following patent documents describe use of glass fibers without defining the type of glass used in making the fibers: U.S. Pat. No. 5,713,972, U.S. Pat. No. 6,340,379, WO 03/095067, WO 99/47236, U.S. Pat. Applications 2002/0023874, 2002/0023419, 2003/0101866, 2004/0163540.
Several patents describe attempts to increase the pollutant adsorption capacity of fibers either by inclusion of absorbants such as activated carbon, zeolites or making the fiber itself porous. Thus, U.S. Pat. Application 2004/0163540, and WO 03/090900, describe use of activated carbon or zeolites, while U.S. Pat. Application 2004/0197552 describes making silica fiber based fabric and treating with a mineral mixture to generate pores on the fiber surface.
Fabrics with improved high temperature filtration capabilities and mechanical properties are needed.